During the last several years, the human T-cell leukemia virus, type I (HTLV-I) has become recognized as an important cause for public health concern throughout the world. HTLV-I is the causative agent of a variety of clinical diseases, including an aggressive lymphoproliferative disorder termed adult T-cell leukemia, and a neurodegenerative disorder termed tropical spastic paraparesis. A large body of evidence indicates that HTLV-I induces pathogenicity in the infected host cell through the synthesis of a protein greatly increases the efficiency HTLV-I gene transcription and replication. Furthermore, Tax expression of certain cellular genes, leading to unchecked growth in the HTLV-I infected cell. The mechanism of Tax trans-activation of gene expression is not known. Tax does not bind directly to target DNA elements with the transcriptional control region of inducible genes, but appears to function interaction with certain cellular DNA binding proteins, such as CREB, that recognize the Tax- responsive promoter sequences. Recent work suggests that Tax may trans- active through enhancement in the DNA binding activity of the target transcription factors, thus increasing promoter occupancy of the activator proteins at Tax-responsive genes. The mechanism of this enhancement is not known. The aim of this proposal is to further characterize activation. Successful in the studies outlined below will provide a better understanding of this potential mechanisms of Tax trans- activation, which is essential to understanding HTLV-I induced pathogenesis. The specific aims described in this proposal are as follows: Specific Aim #1: Determine the mechanism of Tax induced enhancement in DNA binding activity in vitro. In this specific aim, we propose a series of experiments designed to characterize the mechanism of Tax-enhanced binding of CREB to its recognition element. These include equilibrium and kinetic studies of CREB-DNA complex formation in the presence and absence of Tax. We also propose experiments aimed at defining the interaction between CREB and Tax proteins in the DNA binding reaction. Specific Aim #2: Correlate Tax enhancement of DNA binding activity with Tax transactivation. These experiments are designed to study the biological relevance of Tax enhancement of cellular protein DNA binding activity. We propose a series of experiments aimed at correlating Tax trans-activation with enhanced DNA binding activity of the target proteins.